Severe weather to strike central U.S. and Tennessee Valley, a precursor to active March?

A forecast of the jet stream (black arrow), a cold front(blue), and a warm front (red) for this evening from the GFS weather model.
(Centre Meteo UQAM-Montreal)

A powerful storm system will move across the middle of the country today and introduce a chance for severe weather, including tornadoes, over a large area from the southern Plains to the Tennessee Valley. As the dip in the jet stream associated with this disturbance (outlined in black above) moves eastward towards the southern mid-tomorrow, more big storms are possible in that region – though the threat for tornadoes is expected to diminish.

The Storm Prediction Center (SPC) has outlined zones (in yellow) where they believe severe weather is possible both today (left) and tomorrow (right).Severe weather is “expected” in at least some portion of the areas shaded in yellow today (left) and tomorrow (right).
(NOAA SPC)

Upper-level jet stream (black arrows) averaged from January 22 to February 24, 2012. Purple hatched area outlines a region favorable for severe weather within this pattern.
(NOAA ESRL)
The upper-level trough tied to the severe weather potential this week is one of many in a parade of similar systems that, over the past few weeks, has created chilly conditions in the West and Rockies, and milder conditions farther east. This sequence of storms is part of a larger weather pattern (shown to the right) that has recently featured a dip in the jet stream over the Rockies and a southwesterly wind downstream from there.

In general, this pattern is well qualified to produce severe weather in parts of the southeastern quarter of the country (in the region hatched in purple), owing to its ability to organize air masses of very different properties into a volatile mix in that area.

The most recent map of severe weather reports so far observed in 2012, shown to below (to the right), indicates that nearly all of the severe weather has occurred in that zone.

Severe weather reports (tornado –red dots, wind damage – blue dots, large hail – green dots) from Jan 1 through Feb 3, 2012.
(NOAA SPC)
The preliminary tornado count for January is 95, with most of the reports coming after January 22nd. This, by far, exceeds the number we had (16) in January last year – which, by the way, ended up being one of the most active tornado seasons in modern times- and also dwarfs the most recent 3-year January average (17). Yet when viewed within the context of the current state of the global circulation, in particular the phase of ENSO (El Nino/ Southern Oscillation), this year’s fast start to the season is perhaps not so surprising.

According to SPC and the research they reference, there is, in fact, a tendency for more tornadoes in winter during La Nina phases of ENSO (which we are in now). In the La Nina January of 2008, for example, there were a whopping 84 tornadoes.

Given the impressive numbers so far this year, and the fact that La Nina is expected to be in place until mid-spring at least, it might be tempting to think we’re lining up for another big year.

Not so fast. The tornado science also shows that the ENSO phase doesn’t really affect tornado frequency in spring. In fact, according to SPC, there is little or no correlation between ENSO phase and tornado frequency in non-winter seasons. This is largely because there are plenty of other elements to the severe weather season puzzle that need to interact with one another in order to affect tornado production in the United States. ENSO phase is at best a small and mostly unreliable piece.

Ultimately, it will be the intraseasonal changes in the global-scale dynamics (not the AO or NAO or some other statistical artifact of the circulation) that will lead us through our severe weather season. And as we look ahead through March, there are signs that variations in these dynamics, which are undoubtedly related to fluctuations in the Madden-Julian Oscillation lifecycle, will gradually shift the jet stream over North America poleward and resist its recent habit of carving out cool-weather troughs in the Lower 48. This expected, albeit temporary, repositioning of the jet stream shows up well in long-range forecasts for the country, which nearly unanimously predict more ridging aloft and a warmer-than-average March for the nation as a whole.

It is possible that this mode we enter later on in March will not be particularly friendly to severe weather in the big picture. If indeed the jet stream retreats to the north, it will take some of the severe-storm ingredients with it. But even with that in mind, it only takes one outbreak from one trough to rewrite the record books.

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